The present invention generally relates to providing an alloy steel having improved wear, heat, corrosion and oxidation resistant characteristics, and more specifically, to an alloy steel having carbon, silicon and chromium added thereto. The particular alloy is especially useful in the manufacture of internal combustion engine parts, particularly valve seat inserts. The method of producing the alloy is especially adapted to powder metallurgy techniques, but the alloy may be produced by foundry casting techniques.
It is known in the powder metallurgy art to add natural graphite as a carbon source to pure iron or low alloy, low carbon steel powders to modify the properties of the sintered steel parts. Such disclosure is contained in U.S. Pat. No. 4,121,927, assigned to the assignee of the present invention. It is further known to add natural graphite as a carbon source to high alloy or stainless steel powders to increase the wear resistance of the sintered steel parts. The type 440 stainless steels, the tool steels, valve alloys and the wear resistant materials contain carbides that contribute to the properties of the metals. However, the presence of such carbides even in small quantities in the as atomized powder significantly reduces the compressibility and green strength of the metal powder and requires annealing prior to compaction. In fact, as the amount of carbides in the as atomized powder increases, the compressibility and green strength decrease rapidly. In addition, it has been found that increased amounts of silicon in the molten steel prior to atomization cause a severe reduction in compressibility and green strength of the resulting powder.
To provide improved compressibility and green strength of the metal powder, it is part of the present invention to add silicon powder to a metal powder which had a reduced silicon content when atomized. As the present invention also relates to an alloy steel produced by foundry casting methods, the increasing of the silicon content of a molten metal alloy is also considered part of the present invention.
Accordingly, it is an object of the present invention to provide an alloy steel having improved wear, heat, corrosion and oxidation resistance.